This invention relates to a method of impregnating a winding assembly such as a stator of a dynamoelectric machine with varnish and thereafter, drying the winding assembly impregnated with the varnish.
A winding assembly comprising a stator core and one or a plurality of windings wound on the stator core, for example, is impregnated with varnish in electrical equipments such as rotating machines, thereby improving an electrically insulating property, a heat proof and mechanical durability of the winding assembly such as a mechanical deformation. In a general method of impregnating the winding assembly with the varnish in the prior art, the winding assembly is impregnated with the varnish reserved in a vessel so that the varnish is sufficiently permeated into the inside of the winding. Subsequently, the winding assembly is pulled out of the vessel and a superfluous amount of the varnish is removed from the winding assembly by self-dropping. The winding assembly is then put into a heating furnace for a drying process.
In accordance with the above-described conventional method, however, it is difficult to permeate the varnish among strands of the windings and between the windings and inner peripheral faces of core slots in each of which the windings are embedded. Furthermore, an excessive amount of the varnish is removed from the winding assembly when it is pulled out of the vessel. As a result, vacant spaces are left in the windings and between the windings and the inner peripheral faces of the core slots, resulting in problems of reduction in the insulating property and the mechanical strength of the winding assembly.
To prevent the above-described left vacant spaces, the prior art has provided another method wherein the winding assembly is accommodated in a tank and thereafter, an atmospheric pressure in the tank is reduced. The varnish is caused to drop onto the winding assembly in the condition that the atmospheric pressure in the tank has been reduced, thereby impregnating the winding assembly with the varnish.
However, even in the above-described method wherein the impregnation is performed in the vacuum vessel, an excessive amount of the varnish permeated into the winding assembly flows out of it in the drying step for hardening the varnish after the impregnation, thus resulting in the vacant spaces in the winding assembly.